The application created three threads. Note how the threads exited in reverse order
from their creation, which might indicate that had the application had more threads,
the threads would accumulate and consume resources.

To get more interesting information, you could try the following in a different
session:

(dbx) when thr_create { echo "XXX thread $newthread created by $thread"; }
XXX thread t@2 created by t@1
XXX thread t@3 created by t@1
XXX thread t@4 created by t@1

The output shows that all three threads were created by thread t@1,
which is a common multi-threading pattern.

Suppose you want to debug thread t@3 from its outset. You could stop
the application at the point that thread t@3 is created as follows: